A longitudinal, retrospective investigation was made of antimicrobial resistance in bacterial isolates obtained from clinical cases in a small animal hospital between 1989 and 1997. Isolates of Escherichia coli and Staphylococcus species were used as Gram-negative and Gram-positive indicator organisms, respectively, and the annual prevalence of antimicrobial resistance was calculated for each organism to each of nine (for E coli) and 11 (for Staphylococcus species) appropriate antimicrobials, including enrofloxacin. Using a chi-square test for trend, statistically significant, rising trends were identified in the resistance of E coli to amoxycillin (P=0.04), clavulanate-amoxycillin (P<0.01) and streptomycin (P<0.01), and in the resistance of Staphylococcus species to erythromycin (P<0.01). There was an equivocal, rising trend for the resistance of Staphylococcus species to cephalexin. No significant trends were apparent for any of the other 15 organism/drug interactions. The annual prevalence of multiple drug resistance was calculated for E coli, Proteus species, Pseudomonas species, staphylococci and streptococci, but no statistically significant trends were identified.